1. Field of the Invention
The invention generally relates to analog-to-digital converters and more particularly to phase equalization in a dual-channel analog-to-digital converters
2. Description of Related Art
Energy calculations for electric power loads are made by power meters of all kinds. Until recently, electromechanical power meters were exclusively employed in millions of homes and businesses worldwide to monitor the amount of power consumption by a user at a particular location. Such monitoring allows the electricity/power entities to monitor the power (energy) usage of the user for proper billing, load monitoring, servicing, etc. In electromechanical power meters, a series of electrical components as well as mechanical disks, gears, indicators, and dials are used to convert voltage and current into energy. In addition to low accuracy, these electromechanical power meters also require periodic manual calibration and check-ups by field service technicians to ensure that they are operating properly. Electronic meters have recently begun to replace electromechanical meters in monitoring power consumption for homes and businesses. In general, because they rely on digital rather than electromechanical components, electronic meters are more accurate and reliable than their counterpart electromechanical meters. Additionally, through networking, electronic meters allow calibration and monitoring check-ups to be performed from a remote location such as a central office thereby greatly reducing the on-site visits by field service technicians. Finally, due to the deregulation of the electricity market already underway in the United States and Europe, broader range of information on consumers"" power use is needed by competing power suppliers for customizing the billing and servicing plan for each consumer. Due to these advantages, in the near future, electronic meters will likely replace all of the 60 million electromechanical power meters that are in use today in industrial and residential applications. In general, electronic power meters uses sensors such as transformers, etc. to measure the analog current and voltage from the power lines which are then converted into digital words using analog-to-digital converters (ADCs). A power value P is then computed using the converted digital current words and converted digital voltage words according to the equation P=V*I wherein V represents voltage and I represents current. However, the measurement process and conversion process may introduce delays into signals carrying the digital voltage words and digital current words which can cause the signals to be out of phase relative to each other. One technique to equalize the phase change involves making compensation to the sensors. This technique, however, may be expensive because it requires making physical adjustments to passive devices. Another technique to equalize the phase change involves scaling the power output value by a predetermnined scaling factor after it has been computed. This technique is based upon the power equation P=I*V*cosxcfx86 that relates voltage V, current I, and the phase angle xcfx86 between I and V. According to this technique, the power value P is divided by the factor cosxcfx86 to compensate for the phase difference between I and V. This requires prior knowledge of the phase angle xcfx86. However, the phase angle xcfx86 is a function of frequency which may drift over time thereby making the scaling factor cosxcfx86 a variable. As such, the power value computed using a fixed scaling factor cosxcfx86 may be inaccurate under this technique. In addition, an actual phase angle (xcfx86) between the current and voltage may exist as the load becomes less resistive. This also will produce an error in the computed power value.
The energy consumed by a particular electric power load can be calculated according to the following formulas:   E  =            ∫      ti      tf        ⁢                  P        ⁡                  (          t          )                    ⁢              ⅆ        t            
and   E  =            ∫      ti      tf        ⁢                  I        ⁡                  (          t          )                    ⁢              V        ⁡                  (          t          )                    ⁢              ⅆ        t            
The energy calculation can be carried out in a sampled data domain, permitting digital multiplication. The measurement system, including sensors and analog-to-digital converters (ADCs), contributes delays of xcex94V and xcex94I to the voltage and current channels. These delays produce an error in the energy calculation, as illustrated in the waveforms of FIG. 2. The error results in a difference in calculated watt-hours between watt-hours calculated with and without delays. In the past, the sample clock for the ADCs has been shifted, making necessary the design of a complex clock generator, not only for the ADCs but for any filters in the signal paths. For example, see U.S. Pat. No. 5,017,860.
According to the present invention, phase compensation in a dual-channel analog-to-digital converter (ADC) is accomplished by holding conversion results in programmable length registers for controllable time periods. According to one embodiment of the present invention, a dual-channel ADC includes first and second delta-sigma modulators and digital filters, subject to multiple sampling rates for optimizing coarse and fine adjustments of delay. Further according to the present invention, an energy calculation is performed in a sampled data domain, which is implemented using digital multiplication techniques in a delay compensation scheme performed in the digital domain. In particular according to the present invention, the digital data subject to filter processing is delayed by predetermined amounts. According to the present invention, the dual-channel ADC is provided with a programmable channel delay mechanism. With such a controllable delay mechanism, there is no need to provide off-chip compensation in sensors used to receive analog signals of interest. Such an off-chip mechanism is costly and requires burdensome physical adjustment of passive devices. Further, there is no need to scale the energy output after it has been calculated. Calculations according to the present invention moreover are further not limited to just one frequency. According to the present invention there is further no need to shift the sample clock of the ADCs, which would require a complex clock generator not only for the ADC components but for any filters in the signal paths of interest. Further, according to the present invention, a differential delay equal to xcex94I-xcex94V is calibrated and compensated subject to an acceptable time delay for production of a correct energy value. The ADC according to the present invention further oversamples received analog signal at clock rates much higher than the output rate of the ADC, and delays are generated in the downstream filters connected to the ADC""s. Thus according to the present invention, the analog signals are left alone and not adjusted. Instead, the data which comes out of the analog circuitry is treated as normal, and delay circuitry is connected between the filter circuitry in the present embodiment.